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Pdf/17/3/932/5319294/932.Pdf 932 by Guest on 28 September 2021 Research Paper Research Paper GEOSPHERE Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges and the western Transverse GEOSPHERE, v. 17 no. 3 Ranges in California from reconstruction and luminescence dating https://doi.org/10.1130/GES02274.1 14 figures; 2 tables of the Orcutt Formation CORRESPONDENCE: [email protected] Ian S. McGregor and Nathan W. Onderdonk Department of Geological Sciences, California State University–Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90804, USA CITATION: McGregor, I.S., and Onderdonk, N.W., 2021, Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges ABSTRACT consistent with models that attribute shortening across the Santa Maria Basin and the western Transverse Ranges in California from to accommodation of clockwise rotation of the western Transverse Ranges and reconstruction and luminescence dating of the Orcutt Formation: Geosphere, v. 17, no. 3, p. 932–956, https:// The western Transverse Ranges and southern Coast Ranges of California suggest that rotation has continued into late Quaternary time. doi .org /10.1130 /GES02274.1. are lithologically similar but have very different styles and rates of Quaternary deformation. The western Transverse Ranges are deformed by west-trending Science Editor: Andrea Hampel folds and reverse faults with fast rates of Quaternary fault slip (1–11 mm/yr) ■ INTRODUCTION Associate Editor: Jeff Lee and uplift (1–7 mm/yr). The southern Coast Ranges, however, are primarily deformed by northwest-trending folds and right-lateral strike-slip faults with The Coast Ranges of California are deformed by northwest-striking faults Received 15 April 2020 Revision received 16 November 2020 much slower slip rates (3 mm/yr or less) and uplift rates (<1 mm/yr). Faults and folds that accommodate active transpression along the North American– Accepted 20 January 2021 and folds at the boundary between these two structural domains exhibit Pacific plate boundary. This northwest structural grain is truncated to the south geometric and kinematic characteristics of both domains, but little is known by west-striking faults and folds that accommodate north-south shortening in Published online 24 March 2021 about the rate of Quaternary deformation along the boundary. the western Transverse Ranges (Fig. 1). The boundary between these distinct We used a late Pleistocene sedimentary deposit, the Orcutt Formation, as tectonic domains is a diffuse zone ~15–20 km across that facilitates differential a marker to characterize deformation within the boundary zone over the past movement between the two domains. The boundary zone has experienced 120 k.y. The Orcutt Formation is a fluvial deposit in the Santa Maria Basin that historic seismicity and large-magnitude earthquake events, but very little is formed during regional planation by a broad fluvial system that graded into known about the amount of Quaternary displacement on the major faults, a shoreline platform at the coast. We used post-infrared–infrared-stimulated slip rates, or uplift rates. This information is critical for understanding the luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was regional tectonics and topographic development, as well as earthquake risk deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope for the local population and critical facilities in the region, such as the Diablo stages 5e-a paleo–sea-level highstands and regional depositional events. The Canyon nuclear power plant and Vandenberg Air Force Base. deformed Orcutt basal surface closely follows the present-day topography Quantitative description of landforms and young deposits is needed to inter- of the Santa Maria Basin and is folded by northwest-trending anticlines that pret the history of Quaternary landscape evolution and faulting (e.g., Bull, 1985; are a combination of fault-propagation and fault-bend-folding controlled by Kamp and Owen, 2012). Measurements of topographic development and fault deeper thrust faults. Reconstructions of the Orcutt basal surface and forward slip are often dependent on localized deposits, such as fluvial terraces or alluvial modeling of balanced cross sections across the study area allowed us to mea- fans, which can be used to bracket the timing and magnitude of offset and uplift sure rock uplift rates and fault slip rates. Rock uplift rates at the crests of two along individual faults. However, if regionally extensive Quaternary deposits are major anticlinoria are 0.9–4.9 mm/yr, and the dip-slip rate along the blind fault present, deformation over a larger area can be analyzed with a single marker, system that underlies these folds is 5.6–6.7 mm/yr. These rates are similar to which provides a more complete picture of tectonic history with less uncertainty those reported from the Ventura area to the southeast and indicate that the in correlation between local deposits (e.g., DeVecchio et al., 2012). We illustrate relatively high rates of deformation in the western Transverse Ranges are also this approach by using the Orcutt Formation, a regionally extensive late Qua- present along the northern boundary zone. The deformation style and rates are ternary sedimentary deposit, as a marker for investigating recent topographic This paper is published under the terms of the growth and the structures controlling this growth at the boundary between the CC-BY-NC license. Nathan Onderdonk https://orcid.org/0000-0001-7893-061X southern Coast Ranges and western Transverse Ranges in California. © 2021 The Authors GEOSPHERE | Volume 17 | Number 3 McGregor and Onderdonk | Quaternary uplift and fault slip rates in the boundary zone between the Coast Ranges and Transverse Ranges Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/3/932/5319294/932.pdf 932 by guest on 28 September 2021 Research Paper Santa Lucia Mountains Nacimiento F. 120° 30’ W 119° 30’ North America Rinconada F. N SF California Los Osos F. San Andreas F. LA San Luis Range Pt. Buchon San Luis Obispo ! Pacific Ocean West Huasna F. SOUTHERN COAST RANGES 35° 35° N S South Cuyama F. a n Santa Maria R Pt. Sal Foxen Canyon F. a fa e l CHFZ Casmalia Hills M Hosgri F.Z. LHF o u n ta in s Santa Maria Basin Ozena F. Little Pine F. Vandenberg LAF AFB Purisima Hills West Big Pine F. BF Santa Ynez River F. Pine Mtn F. Honda F. Solvang Pt Arguello Santa Ynez F. Area of Figure 2 Sant Santa Ynez F. a Ynez Mountains WESTERN TRANSVERSE RANGES Pt Conception Santa Barbara SCF Santa Barbara Channel RMF Pacific PP- VF Ventura Ocean 0 5 10 20 Kilometers 119° 30’ 34° 120° 30’ 34° Figure 1. Location map of the western Transverse Ranges and southern Coast Ranges showing topography, main geographic features, and faults (red lines). Fault abbreviations include: CHFZ—Casmalia Hills fault zone, LAF—Los Alamos fault, BF—Baseline fault, LHF—Lions Head fault, RMF—Red Mountain fault, SCF—San Cayetano fault, PP-VF—Pitas Point–Ventura fault. Coloring of topography shows relative elevation across the region with light green shades in lower elevation and orange shades in the higher elevations. Other abbreviations: AFB—Air Force Base, F.—fault, Pt.—Point, F.Z.—fault zone. Inset map shows location within California with the cities of Los Angeles (LA) and San Francisco (SF) for reference. GEOSPHERE | Volume 17 | Number 3 McGregor and Onderdonk | Quaternary uplift and fault slip rates in the boundary zone between the Coast Ranges and Transverse Ranges Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/3/932/5319294/932.pdf 933 by guest on 28 September 2021 Research Paper The western half of the boundary zone is a region of low hills and coastal (Page and Engebretson, 1984; Zoback et al., 1987; Hauksson, 1990), initiated plains called the Santa Maria Basin, which is bounded by the Santa Ynez high rates of shortening across the western Transverse Ranges, and caused Mountains on the south and the Santa Maria River Valley on the north (Figs. 1 inversion and uplift of the rocks in the Santa Maria Basin. Shortening and and 2). The Santa Maria Basin formed during Miocene extension and has since uplift in the Santa Maria Basin are accommodated by a system of folds, Mio- been inverted by thrust faults and folds during Pliocene to present shortening. cene normal faults reactivated with reverse slip, and low-angle thrust faults Retrodeformable cross sections and stratigraphic correlations of Miocene and (Namson and Davis, 1990; Gutiérrez-Alonso and Gross, 1997). Shortening in Pliocene horizons across the basin have been used to estimate fault geome- the Santa Maria Basin may have accommodated differential rotation between tries and total convergence (Krammes and Curran, 1959; Namson and Davis, the western Transverse Ranges and southern Coast Ranges (e.g., Lettis et al., 1990; Clark, 1990; Seeber and Sorlien, 2000). However, these studies did not 2004) as well as the southward decrease in slip on the Hosgri fault and its address the Quaternary deformation. We addressed this lack of knowledge termination offshore of Point Arguello (e.g., Sorlien et al., 1999; Dickinson et using the Orcutt Formation to document late Pleistocene deformation and al., 2005). Shortening has continued into Quaternary time, but the amount tectonic history. The Orcutt Formation is a predominantly fluvial, regionally and rate of Quaternary deformation have not been well studied. Long-term extensive unit that was deposited on a low-relief surface that existed between slip rates on the southern Hosgri fault offshore are around 2 mm/yr (Sorlien et the Santa Ynez and San Rafael Mountains (Fig. 2; Woodring and Bramlette, al., 1999), but there are no published Quaternary slip rates on faults within or 1950; Worts, 1951; Muir, 1964; Dibblee and Ehrenspeck, 1989; Clark, 1990). The at the boundaries of the Santa Maria Basin.
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